Alternative near-field gradient probe for the suppression of radio frequency interference

ABSTRACT

A sensor probe. The probe includes a central loop and a plurality of peripheral loops disposed peripherally relative to the central loop. To maximize far-field suppression, current flows in a first direction through the central loop and in a second direction through each one of the plurality of peripheral loops, the first direction opposite to the second direction, and current through the central loop equals current through the plurality of peripheral loops.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Provisional Appln. 62/771,327, filedNov. 26, 2018, the entire contents of which are hereby incorporated byreference as if fully set forth herein, under 35 U.S.C. § 119(e).

BACKGROUND 1. Field of the Invention

Certain of the embodiments described herein relate to RF (radiofrequency) circuits, inductive structures, and related techniques andmethods to improve sensitivity in remote sensing applications thatsuppress the reception of RF interference. This includes methods, andtechniques, as well as systems, methods, and devices for improvingnear-field sensing applications, such as explosive detection and RFIDsystems (radio frequency identification systems).

2. Description of the Related Art

Electromagnetic waves propagating from antenna systems have near-fieldand far-field radiation regions. The near-field is a region near anantenna where the angular field distribution depends upon the distancefrom the antenna. The near-field is generally within a small number ofwavelengths from the antenna and is characterized by a highconcentration of energy and energy storage in non-radiating fields.

In contrast, the far-field is the region beyond the near-field, wherethe angular distribution of the field is essentially independent of thedistance from the antenna. Generally, the far-field region isestablished at a distance of greater than D²/λ from the antenna, where Dis an overall dimension of the antenna that is large compared towavelength λ, the wavelength of the radiating signal. Generally, antennaradiation or the radiating signal is considered to propagate in thefar-field region as a plane wave.

Antennas used to create and exploit the energy in their near-field arefound useful in RFID, nuclear magnetic resonance (NMR), quadrupoleresonance (QR), resonant power transmission, and other applications.Used in this manner, these antennas may commonly be referred to assensor probes. For example, some Radio Frequency Identification (RFID)systems use near-fields for communications between the RFID transpondertag and the RFID interrogator. The energy stored in the near-field mayalso provide the power to drive a microchip embedded in a passive RFIDtransponder tag. RFID systems are typically wireless, non-contactsystems that use radio frequency electromagnetic fields to transferinformation from an RFID card or transponder tag to a reader orinterrogator for the purposes of automatic identification and/ortracking of the item to which the tag is attached.

The sensor probe antennas may be oriented vertically or horizontallydepending on the orientation of the target.

At least some known explosive detectors and RFID systems use loop-typeradiators (i.e., loop-type transmitting antennas) for the interrogatoror transmitting antenna, for example, an antenna consisting of afigure-eight shaped conductor, to reduce the creation of energy in theirfar-field regions. That is, loop antenna systems can be designed suchthat the coupling between the antenna and its nearby surroundings (i.e.,the near-field) is relatively high, whereas the coupling between theantenna and its distant surroundings (i.e., the far-field) is minimized.Since the near-field energy is most important for these sensor probes,such as the probe of the present invention, a minimal far-fieldinteraction is desirable. The relative strength of the near-field isenhanced relative to the unwanted far-field.

To minimize the far-field interaction, two or more loops are used incombination, where the loops may have a specific size and geometry, suchthat the magnitude and direction of the current within the loopsgenerate fields that cancel each other in the far-field region (that is,the vector sum of the fields created from each of the antenna loops isclose to zero). Dependent upon the intended purpose of the system,suppression of interaction with the far-field region can be confined toeither the receive or the transmit function of the system or both can besuppressed.

For example, it is desirable to suppress the far-fields created by thetransmit function of some RFID systems to permit the creation ofstronger near-fields to power passive tagging chips at greater distancesfrom the interrogator. Conversely, when suppression of far-field sourcedRF interference is the primary objective, it is desirable to mitigateinteraction with the far-field in support of receiving weak signalsinitiated in the near-field region of the sensor system.

Specifically, when using a loop antenna system as described above in areceiving mode, energy emanating from the far-field region inducesvoltages in the loops that are equal in magnitude, but opposite inpolarity, such that they sum to zero at the output terminals of thereceiving antenna, while the reception of near-field signals is littleaffected. This far-field suppression is a desirable feature in anantenna for use in sensor probes, such as explosive detectors.

One application for near-field sensor probes (including those usingloop-type radiators as described above) is a detection system thatexploits a material's Nuclear Quadrupole Resonance (NQR), where NQR is aradio frequency (RF) spectroscopic technique for detecting andidentifying a wide range of materials based on detection of theresonances associated with their quadrupolar nuclei. The energytransmitted from a near-field probe excites this resonance in a materialexhibiting this NQR resonance characteristic. The material then radiatesa response signal, which must be detected by the probe's receivingantenna in the presence of radio frequency interference (RFI) in theenvironment and typical Gaussian noise in the receiver. The NQR responsesignal provides a unique signature of the material of interest thatindicates the presence of quadrupolar nuclei in the radiated material.Exemplary uses for NQR detection include (but are not limited to),screening of airline baggage, parcel screening, detection ofdrugs/narcotics, and detection of explosives, such as detection ofburied mines and Improvised Explosives Devices.

One drawback with systems that use near-field probes and relatedtechnologies, especially for detection and screening of explosives, isthe need to operate in the presence of significant RFI, especiallyfar-field RFI. Therefore, some means of suppressing this interferencewithout significantly degrading near-field performance is required.Systems for suppressing far-field RFI are known and at least one isdescribed above (a loop-type antenna where the far-field radiationgenerates opposite-polarity voltages that cancel) but the varioustechniques provide different levels of suppression relative to thedegradation of their sensitivity to nearby (near field) signals. Thatis, the net benefit of achieved RFI suppression is somewhat offset by aloss in signal levels received from the material under test vianear-field radiation. Minimizing this loss in sensitivity, whilemaintaining a suitably high level of RFI suppression offers thepotential to further enhance the performance of the detections system.

Suppression of RFI is particularly relevant for NQR systems, because theresponses are relatively weak signals in segments of the RF spectrumoccupied by high-power radio stations and subject to significantman-made and atmospheric noise sources. Detection of NQR signals, usinga near-field probe (antenna) such as a loop antenna, can be difficult inthe presence of strong far-field noise sources/signals, such as AM radiotransmitters, and nearby noise sources/signals, such as automobileignitions, fluorescent lighting, computers, mobile phones, and otherelectronics devices.

The presence of these strong far-field noise sources/signals presents adifficulty that arises at least in part because these kinds of noisesources can create substantial coherent and non-coherent geographicallydistributed noise that can be within the detection frequency ranges ofinterest. For example, detection of land mine explosives such astri-nitro-toluene (TNT) can be affected by amplitude modulation (AM)radio signals sourced in the far-field, because the characteristicdetectable frequencies associated with TNT (used in NQR detectionsystems) are below 1 MHz, which is within the standard AM radio band.

It is desirable to suppress RFI emanating from distant sources, so thatthis RFI does not interfere with detection of the desired signal. Someknown implementations that attempt to suppress far-field RFI use asingle sensor probe to implement both transmit and receive functions,augmented with a remote RFI sampling antenna (which senses all energy inits vicinity, but is conventionally placed at a location where it willbe less sensitive to the desired target's response to near-field energy)coupled to a weighted negative feedback loop to cancel the RFI andthereby reduce probe susceptibility to RFI. Such a system is referred toas an active cancelation network.

This kind of implementation can introduce undesirable performancecompromises that can lead to performance degradation. In particular, thedesire to maximize efficiency of the receive function works inopposition to the desire to limit the time it takes for the transmitenergy in the probe to dissipate after the transmit pulse has ended.That is, it is preferable to limit the coupling between the transmit andreceive energy in the probe. In fact, overall system performance isfurther improved by separating components associated with the transmitand receive functions so that each can be optimized for its specificfunction.

Additionally, the effectiveness of a remotely located sampling antennais limited because the distributed nature of the RFI causes signals thatare acquired from locations different from the location of the sensingprobe to vary significantly in ways that cannot be fully compensated byadjusting the phase and amplitude of the signal derived from thesampling antenna.

Specifically, the response from the remote sampling antenna(s) does notexactly match the response derived from the sensor probe. Outside of anarrow frequency band, the responses will differ in one or both ofamplitude and phase. Use of remote sampling antennas can also imposestringent linearity requirements on the active components of the probesystem, that is, the first stage of amplification (e.g., a low-noiseamplifier (LNA)) to assure the desired response signal is not lostbecause of saturation of the amplifier before the response signalreaches the RF-interference cancelation stage.

Still other implementations may incorporate shielding over some or allof the probe in an attempt to reduce RFI; this is more common withlarger resonant probes, and can result in bulky probe configurations.Further, such shielding is best suited for detection of buried threats,but is much less effective in personnel screening applications.

Several previously patented inventions (U.S. Pat. Nos. 8,717,242 and8,717,242) and a published patent application of a pending application(US 20150372395 A1) by the current inventor have described means ofsuppressing RFI using a set of properly connected and properly sized,collocated loops. Specifically, two or more smaller loops are collocatedat the center of a larger loop such that the total area of the smallerloops equals the area of the larger loop. The smaller loops areconnected to the larger loop such that the voltage induced in thesmaller loops by any RF interference sourced at a distance, that is inthe far-field, is equal and opposite in polarity to the voltage inducedin the larger loop. Thus, the interconnection of the loops actspassively to minimize the voltage developed due to far-field radiationimpinging on the small and large loops.

However, in this system the fields generated proximate the loops(near-field radiation) do not induce equal voltages in the variousloops. Specifically, the voltage induced by fields generated by sourcesnear the larger loop is significantly less than the voltage generated inthe smaller loops. For this reason, this combination of loops is seen tobe a very effective means of sampling fields nearby, while rejectingfields sourced from substantially greater distances.

While this prior art technique offers a very significant potential tosuppress the interaction with the far-field, whether suppressinggeneration of the far fields as part of a transmit function orsuppressing reception of the far fields in a receive function, it isfound to be fairly costly regarding sensitivity to signals confined tothe near-field region of the probe. That is, this technique reducesnear-field sensitivity to achieve a very similar amount of far-fieldsuppression. Further, in practice, the extremely high suppressionfactors theoretically possible with such co-located geometries have beenfound to be limited by vagaries of the practical implementations of suchsystems. Specifically, practical RFI suppression is found to besignificantly less than six orders of magnitude due to factors beyondthe control of the system design, such as interactions with theenvironment near the system installation.

Therefore, it is the intention of the invention described here toimprove the near-field sensitivity relative to the prior art, whileseeking to maintain the practical level of RF suppression previouslydemonstrated by said prior art approaches.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the embodiments describedherein. This summary is not an extensive overview of all of the possibleembodiments, and is neither intended to identify key or criticalelements of the embodiments, nor to delineate the scope thereof. Rather,the primary purpose of the summary is to present some concepts of theembodiments described herein in a simplified form as a prelude to themore detailed description that is presented later.

In one embodiment, five loops are configured such that a larger centralloop is surrounded by a number of smaller loops (e.g., four) near or atthe periphery of the central loop (and referred to as peripheral loops)and uniformly distributed (or equally spaced) around the central loop.For example, one configuration comprises a square central loop of area Awith four smaller loops each of area A/4 placed adjacent to the cornersof the larger central loop and oriented relative to the central loopcorner-to-corner. Electrical connections are made to the loops such thatthe current flowing in the larger (inner) loop flows in the oppositedirection in the smaller outer or peripheral loops.

The far-field created by a current in a loop having dimensions much lessthan a wavelength is dependent only on its enclosed area and not itsshape. And since the field pattern of such electrically small loops isfundamentally identically independent of the size or shape of the loop,the effect of this geometry is to render a net far-field that issubstantially suppressed. In practice, the level of suppression isalmost entirely dependent on the difference in phase delay between thecenters of the central loop and the peripheral loops. That is, if thespacing is small relative to a wavelength, the suppression achieved issubstantial.

In a second implementation, twice as many smaller loops are sized to sumto the area of the larger central loop (each smaller loop having an area⅛^(th) of the larger loop), but rather than arrayed uniformly around thelarger loop they are placed in pairs at the four corners (for a largerloop that is square-shaped) of the larger loop. That is, nine totalloops where four pairs are positioned at the four corners of thecentral, larger loop. This approach permits use of a larger centralloop, without increasing the plan footprint for the probe. The resultincreases the volume of usable coverage in area and/or distance from theplane of the probe. That is, it results in greater sensitivity tonear-field radiation while negligibly affecting the achievablesuppression of the far-field. In this embodiment, the near-fieldsensitivity at a given distance is greater than for a smaller centralloop, while the maximum phase dispersion is only slightly worse than thefive-loop configuration described above.

In an analogous manner, the number of grouped smaller loops; three, fouror more, can be placed at the corners of the larger loop(s), with theproviso that the smaller loops are sized to yield a composite area equalto that of the larger central loop. These grouped loops can be placedwith their centers arrayed on a line normal to the plane of the probe(i.e., stacked) and/or nested in the same plane as the probe and eachhaving slightly different diameters while maintaining the requiredcomposite area as described elsewhere herein. That is, if eight outerloops are used, for example, each of the pairs can be placed in a singleplane by making one loop slightly smaller than ⅛^(th) of the area of thelarger loop and another loop made slightly larger, such that the sum ofthe two areas is equal to ¼ the area of the larger central loop. Twoouter loops can therefore nest together in a single plane, rather thanrequiring two planes to stack the two loops.

As noted earlier, the shape of the various loops does not have anoticeable effect on the performance of the probe assembled as describedherein. However, particular shapes do offer the opportunity to creategeometries that provide a more efficient packing of the loops within adefined boundary. Because less space in the plane of the probe remainsoutside of an actively excited loop, the probe exhibits a slightlyhigher sensitivity to near-field signals, while potentially reducing thephase dispersal between the inner and outer loops (due to the closepacking of the loops) and thereby improving the potential suppression ofthe far-field interaction. In particular, one preferred implementationuses circular loops for both the inner (larger) and outer (smaller)loops.

Polygons with eight or more sides exhibit very nearly the sameperformance as that exhibited with the use of circular loops.

Further enhancement of the packing efficiency is achieved by enlargingthe central loop out toward or to the boundaries of the space allocatedto the probe in a particular system environment. Such an implementationwould by necessity require the smaller loops to impinge upon or overlapthe area (also referred to as the interior region) occupied by thelarger loop. Assuming the larger central loop is circular or octagonal,the smaller outer loops would need to be placed partially inside andpartially outside that boundary. However, when the shape of theindividual loops is not square, that is a polygonal or a circular shape,the overlap is not total, since in a presumed rectilinear planform,there are regions outside of the polygonal or circular shapes thatremain within the rectilinear boundary. In particular, this approach isbest suited to the use of groups of multiple smaller loops as theirindividual areas impinge on the area defined by the larger loop to alesser degree than for the case that makes use of four single loops.

While it is likely that the smaller and larger loops are chosen to havecongruent shapes, this is not a necessary condition for this approach toachieve the expected performance. The only necessary stipulations arethat the areas of the two types of loops sum to the same value, areelectrically small, carry identical current and are connected in amanner consistent with the desire to reverse the polarity between theinner and the outer loops.

In another preferred implementation, the single central loop is replacedwith two loops of equal size and four outer loops are sized to equal thearea of the two central loops. The outer loops have linear dimensionsthat are 70.7% the dimensions of the central loops and are connected tothe pair of central loops in a manner to reverse the currents flowing inthe four outer loops relative to current flow in the two inner loops.That is, the loops are sized using a factor of 70.7% (1/√2) of eachlinear dimension (height and width) relative to the linear dimensions(height and width) of each of the central loops. Thus, the area of theeach of the smaller loops is one half the area of either of the twocentral loops. Such a configuration exhibits an added feature in thatthe H-field (magnetic field) near the plane of the probe issignificantly higher than for other configurations having the same inputcurrent. Calculations show that these near-fields can exceed those forthe previously described probe having two central loops and a single,larger concentric loop, as well as a higher field-strength farther fromthe plane of the probes. The larger H-field near the plane of the proberesults in a near field with a higher field strength for the same inputcurrent.

It is noted here that the selection of four groups of outer loops (eachgroup comprising two loops) is not completely arbitrary. Using fewergroups of smaller loops, say two or three, results in a certain level ofsuppression of the far-field interaction. In addition, increasing thenumber of groups beyond four is possible. However, fewer than fourgroups, while considered to be included as part of the scope of thisinvention, results in either reduced far-field suppression or offers aninferior area utilization. Increasing the number of groups, while againconsidered to be within the domain of this invention, leads toperformance analogous to that of the previously described system ofloops in U.S. Pat. No. 7,714,791 having collocated centers, upon whichthis invention seeks to improve.

In another embodiment, an independent input port is achieved byintroducing the properly sized conventional single turn loop at theappropriate distance from the differential probes described herein and adevice that is suitable for fine adjustment of the mutual magneticcoupling between said probes is illustrated and described below.

In another preferred implementation a small set of figure-eight shapedloops are used to adjust the amount and polarity of mutual couplingbetween the two probes to establish a maximum level of isolation betweenthem.

Still other aspects, features, and advantages of the invention arereadily apparent from the following detailed description, simply byillustrating a number of particular embodiments and implementations,including the best mode contemplated for carrying out the invention. Theinvention is also capable of other and different embodiments, and itsseveral details can be modified in various obvious respects, all withoutdeparting from the spirit and scope of the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a probe configuration of loops, commonly known as afigure-eight, as known in the prior art;

FIG. 2 illustrates a probe configuration of concentric, coplanar loopsas known in prior art;

FIG. 3 illustrates an implementation of the current invention composedof five square loops for far-field suppression;

FIG. 4 illustrates a probe configuration of the current invention havingfour octagonal outer loops and one octagonal inner loop overlaid on theoutline of an identically sized probe composed of square loops;

FIG. 5 illustrates the relative performance (relative gain in fieldstrength) advantage the three probes configured as in FIG. 4 above withvariously one, two and four outer loops at the four corners of the probecompared to the H-field strength of the prior art probe shown in FIG. 2;

FIG. 6A illustrates a five-loop probe

FIG. 6B illustrates a vector diagram of the resultant H-field created bythe five-loop probe of FIG. 6A.

FIG. 7 illustrates an alternative probe configuration having twoconcentric inner loops and four outer loops and a magnetically isolatedloop;

FIG. 8 shows the sensitivity performance of the probe of FIG. 7 comparedto that of an equally sized probe as shown in FIG. 2;

FIG. 9 illustrates an alternative probe configuration having stackedinner and peripheral loops.

FIG. 10 illustrates an alternative probe configuration having nestedcentral and outer circular loops.

FIG. 11 illustrates an alternative probe configuration wherein octagonalperipheral loops extend into an interior region of a central octagonalloop.

FIG. 12 illustrates a device made of two figure-eight shaped loopsconfigured to provide a mechanism suitable for the fine adjustment ofmutual coupling between two circuits, such as magnetically isolatednear-field probes.

DETAILED DESCRIPTION OF THE INVENTION

A method and apparatus are described for a near-field gradient probe forsuppressing radio frequency interference. In the following description,for the purposes of explanation, numerous specific details are set forthin order to provide a thorough understanding of the present invention.It will be apparent, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope are approximations, the numerical values set forth inspecific non-limiting examples are reported as precisely as possible.Any numerical value, however, inherently contains certain errorsnecessarily resulting from the standard deviation found in theirrespective testing measurements at the time of this writing.Furthermore, unless otherwise clear from the context, a numerical valuepresented herein has an implied precision given by the least significantdigit. Thus, a value 1.1 implies a value from 1.05 to 1.15. The term,“about” is used to indicate a broader range centered on the given value,and unless otherwise clear from the context implies a broader rangearound the least significant digit, such as “about 1.1” implies a rangefrom 1.0 to 1.2. If the least significant digit is unclear, then theterm “about” implies a factor of two, e.g., “about X” implies a value inthe range from 0.5×to 2×, for example, about 100 implies a value in arange from 50 to 200. Moreover, all ranges disclosed herein are to beunderstood to encompass any and all sub-ranges subsumed therein. Forexample, a range of “less than 10” for a positive only parameter caninclude any and all sub-ranges between (and including) the minimum valueof zero and the maximum value of 10, that is, any and all sub-rangeshaving a minimum value of equal to or greater than zero and a maximumvalue of equal to or less than 10, e.g., 1 to 4.

Some embodiments of the invention are described below in the context ofsuppressing far-field interference in an application for detecting andscreening explosives. However, the invention is not limited to thiscontext. In other embodiments the inventions can be employed to suppressfar-field interference in an application where it is desired to measureor sense an attribute of near-field radiation or to suppress thegeneration of far-field radiation when it is desired to maximize theenergy in the near field.

According to the prior art, specific geometries have been designed tomaximize suppression or interaction with the far-field region around asensor probe. This is contrary to the common intention of a radiatingsystem antenna design, where the objective is to maximize the creationof the far-fields This is commonly accomplished by reducing the energypresent in the vicinity immediately adjacent to the antenna structure,as such energy is merely stored in that region and can limit the usablebandwidth of said antenna.

The reverse is the intention of some probes used to excite or sense theregion nearby a sensor probe, as is the case here. In some applications,it is advantageous to significantly reduce the far-field radiation fromthe probe, such that the energy in the near-field can be maximized. Thisis especially true for some RFID systems, specifically those operatingin the 13.56 MHz ISM band. Reciprocally, some applications benefit fromthe suppression of the reception of energy sourced far from the sensorprobe so that weaker signals nearby the probe are not masked.

One common approach to suppress interaction with the far-field region ofa sensor probe is to institute a probe geometry such that the energy inone or more parts of the probe is subtracted from the energy present inanother part of the sensor probe. Thus, the resultant energy is thedifference of the energy in the parts as opposed to the sum. Thisresults in a probe that is responsive to the gradient (rate of change)of the field created by the probe and the far field energy that impingesupon the probe. Energy emanating from a great distance from the probe isvery substantially planar (a plane wave) and therefore has almost nogradient. However, a penalty is paid in instituting such a differencingcondition in the probe in that some loss in sensitivity resultsthroughout the region around the probe, i.e., the near field region. Bythe antenna reciprocity theorem, the differencing probe condition alsosuppresses the near-field radiation created for a given input current(power). It is therefore beneficial to minimize the penalty incurred,while achieving a desired level of suppression of the far-fieldinteraction (i.e., both generating far-field energy and receivingfar-field energy).

Also, according to the antenna reciprocity theorem, any approach thatsuppresses far-field reception in a receive application also suppressescreation of far-field radiation in a transmit application.

In possibly the simplest embodiment of prior art which achieves somelevel of far-field suppression is pictured in FIG. 1. It comprises twoequally sized small loops 110, and 120 positioned in a single plane sideby side. The current 120 from the feed point 100 is seen to progresscounter-clockwise through the right loop and reverses direction toprogress clockwise in the left loop. In this way the voltages induced bythe fields far from the pair of loops are substantially the same andopposing and therefore suppressed. This is the common basis for all suchfar-field suppression approaches. That is, the imposition of opposingfields such that they tend to cancel at a distance great enough from theloops that the fields emanating from each of the separate parts (loops)of the probe are very substantially equal and opposite, that is, at adistance in the far-field region of the probe. While this geometry doesachieve some level of suppression of the far-field interaction (i.e., inboth transmitting and receiving modes), it also significantly degradesthe performance in the nearby or near-field region both in area coverageand in sensitivity.

In a prior art field suppression device 1100 pictured in FIG. 2, the twosmaller inner loops 1110 and 1120 are connected to the one outer loop1130 in a manner at a junction through couplings 1140 and 1142 to causethe currents in the loops to flow in opposite polarities (clockwisedirections). Note that all of the loops are contained within a singleplane 1150. The area of the two smaller loops 1110 and 1120 are sized sothat the sum of their areas equals the area of the larger outer loop1130. Since the three loops are connected in series, the same currentflows in each of the loops. Therefore, the current-area products of thelarger and smaller loops are equal and current flows are opposite, whichis a necessary condition to suppress the far-field interaction.

The objective of this geometric configuration in FIG. 2 is analogous tothat of the prior art of FIG. 1, but has the added advantages ofcentralizing the strongest fields and minimizing the phase dispersionbetween the loops. The lack of significant phase dispersion maximizesthe suppression of the far-field interaction, while the centering of thefield acts to limit somewhat the degradation of the near-field thatcommonly occurs with such differential loop geometries.

However, it has been shown by experience that the practical suppressionof the far-field interaction has a limit unrelated to the details of thedesign of such differential probes. It is presumed that this limitarises because the fields generated by the probe in a transmit mode orimpinging upon it in a receive mode are not perfectly plane due toenvironmental factors beyond the control of the probe's design orgeometry. Another contributing factor could be that perfect shielding ofperipheral equipment necessary to create a full system implementationcannot be achieved. Therefore, in practice it is found that suppressingthe far-field interaction beyond five orders of magnitude (50 dB) isdifficult and reliably beyond six orders of magnitude (60 dB) isimpractical. In light of this understanding, the current invention actsto maintain a practical level of far-field suppression, while reducingthe amount of near-field degradation from that exhibited in the priorart implementations.

A fundamental implementation of the current invention is pictured in theembodiment of FIG. 3. The probe consists of square shaped loops with acentral larger loop 210 and four identical smaller loops 220 arrayeduniformly and attached at the corners of the larger loop 210 throughcrossover interconnections 240. The impressed current 230 fed at asomewhat arbitrary feed point location, here placed at a convenientlocation at the lower left corner 200, is seen to flow through thelarger loop 210 in an opposite clockwise direction from the direction ofcurrent flow through the four smaller loops 220. Note that the geometryis selected to fulfill the necessary condition of the sum of thearea-current product of the smaller loops 230 being equal to thearea-current product of the larger loop 210. Specifically, the smallerloop linear dimensions are one-half those of the larger loop. A dashedline 250 depicts the rectilinear boundary of the probe.

Note that if the current in the inner loop is not equal to the currentin the outer/peripheral loops, maximum far-field suppression is achievedwhen the product of the area and current in the inner loop is equal tothe product of the area and current (flowing in an opposite direction)in the outer/peripheral loops (referred to herein as the area-currentproduct).

The shape of the various field suppression loops has almost no effect onthe performance of the probes assembled as described herein. However,certain shapes do offer the ability to improve the packing efficiency ofthe loops within a defined boundary, that is, specifically to enclose agreater fraction of the area within a given boundary within the activecentral loop. The advantage of such geometries is threefold: first thephase dispersion between the various loops is decreased (where the phasedispersion limits far-field suppression); second the greater areaincreases sensitivity (higher H-field created for a given current whentransmitting or higher voltage induced for a given field strength whenreceiving); and finally, the volume of coverage is increased.

An example of the advantage provided by other than a square shape isdepicted in FIG. 4, where the outline of a five-loop version of theinvention (solid line 300) composed of octagonal loops is superimposedover a five-loop version of square loops (dashed line 310). The feedpoint of the sensor probe and interconnections between the central andperipheral loops are omitted for clarity, but the opposing relationshipbetween the current in the larger (central) loop and the four outerloops is assumed.

Two distances in FIG. 4 are marked by arrows 320 and 340 between thegeometric (phase) centers of the central loop (both the octagonal andsquare central loop) and the geometric (phase) center of one of theouter loops (both the octagonal and square outer loops) to illustratethe reduction in phase dispersion between the two embodiments (squareand octagonal). Phase dispersion is dependent on the difference inarrival times at the constituent parts (loops) of the probe, as sourcedin the far field. The arrowhead 330 (for the octagonal embodiment) is ofidentical length as that of the arrowhead 320, repositioned to alloweasy visualization of the differences in length. For a square boundaryas shown (arrowhead 340), the distance between the phase centers isreduced by 10% for the octagonal geometry relative to that of the probeconfigured with square loops. While not great, it is tending in afavorable direction. Decreasing the distance between the inner and outerloops has the effect of increasing the far-field suppression for a givensized probe. Therefore, the phase dispersion that can be tolerated setsthe maximum size of the probe at a given frequency (wavelength) thatmeets the acceptable far-field suppression level.

It is also noted that the octagonal loop design results in an activearea that is nearly 36% larger than that achieved with the square shapedloops. This translates directly into a greater field strength andamounts to a more than 2.5 dB increase in sensitivity to the near fieldradiation. An additional gain is achieved through the employment of anine-loop configuration consisting of one central loop and eight smallerouter loops distributed in pairs at the corners, as with the five-loopconfiguration. As with other embodiment, the sum of the areas of theloop pairs is set equal to the area of the central loop. Since this isachieved with loops having smaller outer dimensions, the size of thecentral loop is increased within the original planform of the probe.This results in an area increase of nearly 42% relative to the five-loopconfiguration.

The gain in sensitivity is characterized here as the relative increasein H-field along a line normal to the center of the plane of the probewhen compared to that of the probe configuration of FIG. 2. FIG. 5illustrates this gain for three large central loop probe configurations:One consisting of four smaller outer loops (as shown in FIG. 4), anotherwith four pairs of outer loops (nine total) and a third having four setsof four outer loops (17 total). It is assumed each loop caries the samecurrent.

It is noted that a slight reduction of sensitivity occurs for the largecentral loop configuration compared to one having two central loops anda single concentric outer loop (see FIG. 2) at distances less than about25% of the size of the probe. However, this is not generally the mostinteresting region around such a probe. Rather, regions out to or beyond100% of the size of the probe are of greater interest. This isespecially true when two probes are arranged in an opposing spaced-apartrelation to form a sensor gate for screening a target (such as personnelor vehicular) that is disposed between the two probes. In this case thegain is fairly modest at about 2 dB for the five-loop probe, but growsto nearly 6 dB for the 17-loop configuration. The gain results in adecrease in the time to detect contraband on the target by up to afactor of four. Such a timing improvement greatly enhances systemperformance by increasing throughput, increasing probability ofdetection, and/or decreasing false alarm rate.

As mentioned earlier, phase dispersion between the various loops of thecentral loop probe configuration limits the absolute level of achievablefar-field interaction. The amount of phase difference between thecentral loop and the outer loops is determined by the distance betweenthe phase center of the various loops. Note that geometrically, thephase center is at the center of a regular polygon or a circle. Forother shapes, the phase center is located at the center of gravity ofthe shape.

The greatest amount of dispersion experienced by the probe configurationof FIG. 4 is along a line 481 in FIG. 6A, which passes through thecenters of central loop, 401, and the two outer loops, 431 and 441, orthe sets of outer loops at opposite corners of the central loop.

FIG. 6B illustrates a resulting vector diagram of a five-loop probe ofthis design. In it, the five-component field amplitude/phase vectors arerepresented by the arrows 400, 410, 420, 430 and 440, laid end to end.These correspond to fields from the loops 401, 411, 421, 431 and 441,respectively. The amplitude of the vector 400 is four times that of theother four vectors. Together, their fields sum to the plane wave frontrepresented by the dashed line 480 in FIG. 6A.

Reciprocally, the vectors can be taken to be the amplitude and phase ofthe voltages at the feed point of each loop resulting from the presenceof an incident plane wave arriving from the angle in question. Theresulting vector 460 is the resultant from the summation of the fieldsof the four outer loops, 411, 421, 431 and 441. This vector isreproduced and repositioned as line 465 to illustrate the effect ofsubtracting the results of the two sets of loops (inner and outer). Thesmall arrow, 470, illustrates the effect of combining the results of thetwo sets of loops.

The angle, ϕ at 450, is the dispersion angle of the field from loops431, which leads that of the central loop, 401, and also the angleresulting from loop 441, which lags that of the central loop. Theseangles result from the time it takes to travel the two distances labeled451 and 452 respectively. It is also noted that the resultants of theloops 411 and 421, and illustrated by rays 492 and 493, are in phasewith the ray from the central loop, 491. The amplitudes of the fourouter loops, 411, 421,431 and 441 are one quarter of that of the centralloop.

Replacing sets of loops at the corners, say two, three, four or moreloops, results in a nearly identical vector diagram, where the sumfields of the sets of loops is substituted for that shown here for thefour-loop configuration. The only difference being the value for thephase dispersion angles, which depends on the spacing between the outerloops and the frequency of operation. That is the phase, ϕ, is equal tothe distance, 451 or 452 in this case, divided by the wave length atthat frequency.

Expressed mathematically, the case pictured in FIG. 6A is given by EQ 1.

$\begin{matrix}{{Result} = {1 - \frac{1}{2} - {\frac{1}{2} \cdot {\cos\left( {2\mspace{14mu}\pi\mspace{14mu}{S \cdot \frac{f}{c}}} \right)}}}} & \left( {{Eq}\mspace{14mu} 1} \right)\end{matrix}$Where S is the spacing between the loop phase centers, f is theoperating frequency, c is the speed of light, and “Result” is a measureof the field suppression relative to the conventional loop. The phasedispersion is defined by the terms in parenthesis.

Using this equation, the theoretical far-field suppression factor atvarious frequencies as a function of frequency can be computed.Conversely, the maximum probe size can be determined at a givenfrequency such that a certain level of cancellation is theoreticallyachievable. For example, it is found that a planform in excess of 1.25meters square yields an expected suppression factor of 60 dB or greaterat a frequency associated with the nuclear quadrupole spectral responseof the explosive material RDX (3.6 MHz). A suppression level of 50 dB isachievable with a probe that exceeds two meters.

But note that phase dispersion increases with the size of the probe. Ifthe desired worst-case suppression is about 60 dB or better, then theprobe should not be larger than 1.25 meters. If a suppression level of50 dB can be tolerated, then the probe can be as big as 2 meters across.These parameters are applicable to a specific frequency since theequation incudes a frequency term.

The results are scalable to any frequency using an electrical length (indegrees). The two lengths given in the example, converted to degrees,are 5.4 degrees and 8.6 degrees. These same suppression levels can beachieved with even larger apertures for operation at lower frequencies.Thus, it is seen that the probe configurations described here do notimpose significant size restrictions relative to those associated withprior art, especially when compared to practically achievable far-fieldsuppression levels previously described here.

An alternative implementation, as illustrated in FIG. 7, results insubstantially the same suppression levels as for the previouslydescribed probe of an equal size, but does not suffer from the loss ofnear-field sensitivity in the region immediately adjacent to the planeof the probe. This is achieved while maintaining a similar sensitivityto that of the five-loop configuration described above.

This alternative implementation of FIG. 7 employs two, equally sized,octagonal central loops, 500 and 510 surrounded by four properly sizedouter loops, 520, 530,540 and 550. The two central loops are positionedin two planes (stacked) having a small separation from the plane of thecomplete probe. For example, in a first embodiment one of the two loops500 and 510 is in the plane of the probe and the other of the two loopsis placed a small distance from the plane of the probe. In a secondembodiment both of the two loops 500 and 510 are placed a small distancefrom the plane of the probe. Further, one of the central loops 500 and510 can be placed in the plane of the outer loops. This stackedarrangement is depicted in FIG. 7 where the loops 500 and 510 are shownin an offset orientation to convey the stacked arrangement.

Reference character 570 in FIG. 7 identifies a magnetically isolatedprobe loop fed at a feed point 580 for generating an incidenttransmitted signal to a target. Depending on the application, such amagnetically isolated loop may be used with other embodiments describedherein.

In one embodiment a width of the central loops is about 52% of the widthof the complete probe, while the width of each of the four outer loopsis about 37% of the width of the complete probe. The maximum phasedispersion distance is denoted by an arrowhead 560.

The estimated sensitivity performance of the FIG. 7 configuration isshown in FIG. 8. This figure compares the H-field created for theimplementation of FIG. 7 when compared to that of the design depicted inFIG. 2. Both probes are assumed to have the same outer planformdimensions and are driven with equal currents.

Like FIG. 7, the embodiment of FIG. 9 depicts both the central loops 500and 510 in a stacked orientation. Each pair of peripheral loops, such asthe pair identified by reference characters 565 and 566, are also shownas offset to represent a stacked orientation.

Yet another approach places both loops in the same plane, but alterstheir size slightly, i.e., one larger and one smaller, to accommodateconcentric placement. This nested arrangement is illustrated in FIG. 10where a smaller inner central loop 570 is nested within a larger outercentral loop 575. Each of the four peripheral loops also comprises asmaller inner peripheral loop (such as the smaller inner peripheral loop585) and a larger outer peripheral loop (such as the larger outerperipheral loop 580). Like the other described embodiments, a sum of theareas of the eight peripheral loops equals the sum of the areas of thetwo central loops to maximize far-field suppression (assuming equal andoppositely-direct currents through the peripheral and central loops).

In still another embodiment the peripheral loops extend into or overlapan interior region of the central loop. See FIG. 11, where peripheralloops 590, 591, 592, and 593 overlap into an interior region of acentral loop 595.

Note that because of the differential nature of all the probeimplementations presented herein, it is possible to introduce a secondmagnetically isolated probe loop that creates an independent probe portfor transmitting an interrogation signal to a target, such as the probeloop 580 in FIG. 7. Conventionally, this probe loop comprises a singleturn. The geometry of the various loop configurations presented causesthe magnetic fields to cancel, and thereby the coupling between the twomagnetic fields to cancel. The coupling has little or no effect on thecapacitive (electric field) coupling, but that coupling factor istypically small for loop antennas. The isolation of this added probeloop is achieved through the proper sizing and placement in closeproximity to any of the differential probes described above. Forexample, a square single turn loop probe properly sized and offsetslightly behind or in front of the differential probe could be used tointroduce a transmit signal. Specifically, applying tuning to match theprobes to their source or load is easily achieved because of theisolated nature of the two ports. Adjustment of one tuning network hasno noticeable effect on the tuning of the other when the proper geometryis implemented.

Such isolation is achieved by adjusting either the size (dimensions) ofone of the loops at a fixed distance between the planes of the twoprobes and/or by adjusting the spacing between the two probes for afixed size.

In another preferred implementation a small set of figure-eight shapedloops are used to adjust the amount and polarity of mutual couplingbetween the two probes to establish a maximum level of isolation betweenthem. While generally, this isolation device can be used with any twoprobes, in the context of the present invention the device is locatedbetween the receive probe and the isolated second probe used for thetransmitter.

Such a device is pictured in FIG. 12. The illustrated device comprises afixed figure-eight shaped lower portion, 600, having a feed point at 610and a moveable upper portion, 630, with a feed point near its center ofrotation, 640. The crossover of the figure-eight in the lower portion isthe jumper 620. Not shown are the jumpers to the second circuit/probe atthe top center of the upper plane/probe 630. The plane of rotation forthe upper portion, 630, is illustrated by the arc 650. Connecting thefixed portion in series with the first probe and the moveable portionwith flexible jumpers to the second probe permits the sum of the mutualcoupling between the first and second probe to be adjusted. If it isassumed that the isolated probe is for the transmit function, the firstprobe is for the receive function. This variable inductor geometry actsto reduce radiation of or reception of nearby fields.

Rotating the moveable portion changes the amount of mutual coupling thatoccurs between the two circuits. As shown in the FIG. 12, the couplingis the maximum that can be achieved between the two halves. Rotating theupper portion 90 degrees in either direction (clockwise orcounter-clockwise) results in the minimum amount of coupling between thetwo halves. Continuing to rotate to 180 degrees results in the maximumamount of coupling between the two halves, but reverses the polarity(sense) of the coupling between the two halves. Thus, the full excursionin mutual coupling is achieved with just 180 degrees of rotation of theupper portion. Further the total coupling between the two circuits towhich this device is connected is thereby adjustable such as to minimizethe resultant coupling.

Although the various embodiments described and illustrated herein havedepicted the various loops of the sensor probe in a horizontalconfiguration, the loops may also be oriented in a verticalconfiguration. The orientation of the loops necessarily depends on theorientation of the target to be probed. If the target is vertical, astanding person for example, the loops of the sensor probe are orientedvertically.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. Throughout thisspecification and the claims, unless the context requires otherwise, theword “comprise” and its variations, such as “comprises” and“comprising,” will be understood to imply the inclusion of a stateditem, element or step or group of items, elements or steps but not theexclusion of any other item, element or step or group of items, elementsor steps. Furthermore, the indefinite article “a” or “an” is meant toindicate one or more of the item, element or step modified by thearticle.

What is claimed is:
 1. A sensor probe comprising: a conductive centralloop; a plurality of conductive peripheral loops disposed peripherallyrelative to the central loop; the central loop and the plurality ofperipheral loops configured such that current flows in a first directionthrough the central loop and in a second direction through each one ofthe plurality of peripheral loops, the first direction opposite to thesecond direction, and wherein current through the central loop equalscurrent through the plurality of peripheral loops; and wherein theplurality of peripheral loops are equally spaced about the central loop,and wherein far-field suppression is responsive to a distance between acenter of the central loop and a center of one of the plurality ofperipheral loops, such that minimizing the distance increases far-fieldsuppression.
 2. The sensor probe of claim 1, wherein an area of thecentral loop is equal to a sum of an area of each one of the pluralityof peripheral loops.
 3. The sensor probe of claim 1, wherein a productof an area of the central loop and the current through the central loopequals a product of an area of the plurality of peripheral loops and acurrent through the plurality of peripheral loops.
 4. The sensor probeof claim 1, wherein a shape of the central loop, and a shape of each oneof the plurality of peripheral loops comprises a same closed geometricfigure, the same geometric figure further comprising a polygon or acircle.
 5. The sensor probe of claim 1, wherein the plurality ofperipheral loops comprises four, five, nine, or seventeen peripheralloops.
 6. The sensor probe of claim 1, wherein the plurality ofperipheral loops comprises a first, second, third, and fourth peripheralloop, the sensor probe further comprising a fifth, sixth, seventh, andeighth peripheral loop each paired with a respective first, second,third, and fourth peripheral loop, wherein a sum of an area of each oneof the first through eighth loops equals an area of the central loop. 7.The sensor probe of claim 1, wherein the central loop and the pluralityof peripheral loops are each defined by a perimeter and oriented suchthat a perimeter segment of each peripheral loop is proximate aperimeter segment of the central loop, and the peripheral loops arespaced equally around the central loop.
 8. The sensor probe of claim 1,further comprising a magnetically isolated loop for transmitting anincident signal directed at a target and receiving a reflected signalfrom the target, the magnetically isolated loop comprising feed pointsfor receiving a current producing the incident signal.
 9. The sensorprobe of claim 1, wherein a distance between a center of the centralloop and a center of each one of the plurality of peripheral loops isless than about 5% of a wavelength of signals impinging the sensorprobe.
 10. The sensor probe of claim 1, wherein the plurality ofperipheral loops comprises eight peripheral loops configured in fourstacks of two peripheral loops, and wherein the loops of each stack areconcentrically oriented, and wherein each stack is oriented such that aperimeter segment of each stack is proximate a perimeter segment of thecentral loop, and the stacks are spaced apart equally around the centralloop.
 11. The sensor probe of claim 1, wherein the plurality ofperipheral loops comprises eight peripheral loops configured in fourgroups of two peripheral loops, wherein within each group a firstperipheral loop is nested within an interior region of a secondperipheral loop, each group oriented such that a perimeter segment ofeach group is proximate a perimeter segment of the central loop, and thegroups are spaced apart equally around the central loop.
 12. The sensorprobe of claim 1, wherein an interior region of each one of theplurality of peripheral loops overlaps an interior region of the centralloop.
 13. The sensor probe of claim 1, wherein a shape of the centralloop is congruent with a shape of each one of the plurality ofperipheral loops.
 14. The sensor probe of claim 1, wherein the pluralityof peripheral loops comprises four peripheral loops, and wherein ageometric figure of the central loop and the four peripheral loopscomprises a square, each of the four peripheral loops orientedcorner-to-corner relative to the central loop.
 15. The sensor probe ofclaim 1, wherein the central loop comprises two equally-sized centralloops and the plurality of peripheral loops comprises four peripheralloops, wherein an area of the two central loops is equal to a sum of anarea of each one of the four peripheral loops, and wherein currentthrough the two central loops is equal to and flows in an oppositerelative to current flow in the four peripheral loops.
 16. A componentfor providing a variable terminal inductance: a first conductive elementin a figure-eight configuration spaced apart from and parallel to asecond conductive element in a figure-eight configuration; the firstconductive element connected in series with the second conductiveelement; the first conductive element having a discontinuous segmenttherein, first and second terminals formed at ends of the discontinuoussegment; and wherein the first conductive element is rotatable in aplane of the first conductive element relative to the second conductiveelement for changing a mutual inductance between the first and secondconductive elements and thereby changing a terminal inductance at thefirst and second terminals.